The E2F family of transcription factors is regulated by pRb and is characterized by the fact that DNA tumor virus oncoproteins alter the association of E2F with the pRb family of tumor suppressors. pRb is able to bind and inactivate E2F; this ability correlates with the ability of Rb to suppress cell growth, indicating that pRb mediated suppression of tumor growth may be mediated by E2F. The adenovirus E1A oncogenic protein also acts via pRb; binding of E1A to Rb releases E2F. Consistent with this view, the E2F transcription factors directly regulate the transcriptional activity of many genes involved in controlling the cell cycle and cell growth, such as c-myc, c-myb, DHFR, thymidine kinase. In order to study the function of E2F, the principal investigator has established an experimental system of 32D cells overexpressing E2F-1; these cells are normally dependent on IL-3 and will differentiate along the myeloid pathway in the presence of G-CSF. Preliminary data show that overexpression of E2F-1 accelerates apoptosis upon IL-3 withdrawal and co-overexpression of DP-1 (a heterodimer partner of E2F-1) augments the rate of cell death. High levels of E2F/DP-1 can induce apoptosis even in the presence of IL-3. Furthermore, preliminary data suggest that p53 protein accumulates and BCL-2 decreases as a function of E2F activity in 32D cells. It is hypothesized that in the context of Rb and P53 inactivation, deregulation of E2F may provide growth advantage and oncogenesis and also interfere with differentiation. In addition, the principal investigator has found that E2F3 and DP1 mRNA levels are regulated by IL-3 in 32D cells, and thus he proposes to determine the role of these genes in myeloid differentiation, to link the cytokine signaling with nuclear events regulating cell cycle progression. Primary cultures of hematopoietic progenitor cells will be used to test the hypothesis that suicide pathways must be inactivated prior to E2F activation in multistep tumorigenesis.